1. Field of the Invention
The present invention relates to a nonvolatile semiconductor storage device and a method for manufacturing the same, and particularly to a nonvolatile semiconductor storage device which stores data by storing charges in a charge storage film and a method for manufacturing the same.
2. Description of the Related Art
Conventionally, a NAND flash memory and a NOR flash memory have been widely used as a nonvolatile memory. Further, there has been recently proposed a flash memory having the characteristics of both of the NAND flash memory and the NOR flash memory. As the NAND flash memory, while a memory using a floating gate had been mainly used conventionally, a memory having a metal-oxide-nitride-oxide-silicon (MONOS) structure has been recently developed.
In the NAND flash memory having the MONOS structure, a charge storage film is formed of insulating films, and data is stored by controlling thresholds of memory cell transistors constituting memory cells. Here, the threshold voltages are controlled by injecting charges into trap sites formed in the charge storage film, and by erasing the charges stored in the tarp sites (see, for example, Japanese Patent Application Publication No. Hei 11-312795).
In recent years, to increase capacity and reduce cost of a nonvolatile memory, individual memory cells have been made smaller and smaller. Further shrinking of size of memory cells in the future will shorten spaces between memory cells and may possibly lower their reliability of holding charges. Further, due to shrink size of memory cells, damage caused during processing of each component may adversely affect the characteristics of the memory cells more seriously.